Digital wireless communication systems use many types of modulation methods in order to transmit data. Generally, in communication standards, such as Bluetooth, Wireless M-Bus and Wi-SUN, a modulation method, such as Gaussian frequency shift keying (FSK), is used. Gaussian frequency shift keying is designed to pass an input frequency-shift keying signal first through a Gaussian filter. Since the spectrum of a frequency-shift keying signal occupies a considerably wide band due to the characteristics of a frequency-modulated signal, the bandwidth of an output signal is limited to a desired level by passing the frequency-shift keying signal through the Gaussian filter. The bandwidth of a Gaussian filter is normally represented by a BT product. The Bluetooth Classic and Bluetooth Smart standards recommend the use of a Gaussian filter having a BT (BT product) value of 0.5.
Furthermore, differential discriminators that are simple to implement are chiefly used for the demodulation of Gaussian frequency-shift keying signals. In the application fields of Bluetooth Smart, the improvement of conventional differential discrimination receivers is required in order to improve receiving sensitivity or extend coverage. Research into a maximum likelihood sequence estimation (MLSE) receiver is being widely conducted in order to improve receiving sensitivity. Although the MLSE receiver has slightly high complexity, it can achieve the improvement of receiving sensitivity equal to or greater than 4 dB compared to a receiver using a conventional differential discriminator.
An example of a representative receiving method for an MLSE receiver for receiving a Gaussian frequency-shift keying signal is a method of receiving a signal by representing a GFSK signal as the sum of pulse amplitude-modulated signals via Laurent's decomposition and obtaining MLSE results through the application of the Viterbi algorithm to matched filters for the pulse amplitude-modulated signals and the output values of the matched filters. According to Laurent's decomposition, an arbitrary phase-modulated signal can be represented by using a modulation index h. Since an MLSE receiver processes a signal on the assumption that the modulation index h is a specific value, a problem arises in that it is not easy to recover a signal when the difference between an actual modulation index and the assumed value is large.
In view of the fact that variations in modulation index allowed in the Bluetooth Classic standard range from 0.28 to 0.35 and variations in modulation index allowed in the Bluetooth Smart standard range from 0.45 to 0.55, the conventional GFSK-MLSE receiver absolutely requires accurate modulation index estimation.
An example of a preceding technology for implementing an FSK-MLSE receiver in a simple form is disclosed in Korean Patent No. 10-0544245 entitled “Device for Receiving and Recovering Frequency-Shift Keyed Symbols.”
FIG. 1 shows a device for receiving frequency-shift keying symbols according to the conventional technology. The device of FIG. 1 includes an antenna configured to detect a Gaussian frequency-shift keying signal from a transmitter. The signal received via the antenna is transferred to a frequency down converter 110 configured to down-convert the received signal. The frequency down converter 110 is connected to a four-order selective filter 120 to be selectively tuned to a desired channel.
The device further includes a selective filter 120 configured to selectively pass a desired channel therethrough and remove an undesired channel. To improve selectivity and the removal of an adjacent channel, the BT of the selective filter 120 is preferably designed to be about ½ of the BT of a transmitter filter (i.e., assuming that the BT of the transmitter filter is 0.5, the BT of the selective filter is 0.25). When the bandwidth of the selective filter is excessively narrow, serious interference is caused between ISI symbols, and thus compensation must be performed when symbols are recovered later. A discriminator 130 is connected to the selective filter 120, and converts received frequency domain symbols into time domain symbols. A symbol recovery processor 140 is connected to the discriminator 130, and recovers the symbols via a 2-state MLSE technique.
In this case, the conventional discriminator 130 generates a voltage signal proportional to the deviation by which the frequency of a signal deviates from the center frequency of the signal, and may have the function of converting a frequency domain signal, having a variation in the frequency domain even when it is a baseband signal, into a time domain signal.
In the conventional technology, both the selective filter 120 and the MLSE-based symbol recovery processor 140 can be easily designed in the state in which the modulation index h has been assumed. Accordingly, the conventional technology is problematic in that a reduction in performance may occur when the estimation of the modulation index h fails.
The conventional GFSK-MLSE signal recovery method is problematic in that a reduction in performance occurs inevitably due to an allowable variation in modulation index that is prescribed in the Bluetooth standard. Another problem of the conventional technology is that it is not easy to determine a modulation index via channel estimation.